vkernel can not access debug registers

I mistakenly pressed 'h' to get help in the vkernel ddb and got a bus error.

ddb interprets 'h' as 'hwatch' (set hardware watchpoint). This calls
db_md_set_watchpoint() which in turn calls fill_dbregs(). The latter
tries to access debug registers which triggers a GPF as the vkernel
runs in user mode.